Transmission line pulse method for measuring electrostatic discharge voltages

ABSTRACT

The present invention relates to a method which introduce a parasitic series resistance for solving electrostatic discharge voltages by using transmission line pulse method and least square error solution method. In present invention, we introduce a parasitic series resistance, Rs, into the equation which presents the correlation between the transmission line pulse method and human body model. The equation is then rewritten as  
     electrostatic discharge voltage=electrostatic discharge current×(the human body equivalent resistance+the parasitic series resistance)  
     We can obtain the optimal parasitic series resistance and electrostatic discharge voltage by using the least square error solution method. By making a comparison between the electrostatic discharge voltage obtained from transmission line pulse method and that obtained from human body model test method, we can find that the correlation of the transmission line pulse method and human body model in this invention is better than that in conventional method.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention generally relates to a method for measuringelectrostatic discharge voltages by using transmission line pulsemethod, and in particular to a method which introduces a parasiticseries resistance for solving electrostatic discharge voltages by usingtransmission line pulse method and least square error solution method.

[0003] 2. Description of the Prior Art

[0004] In integrated circuit manufacturing and handling environments,there are three principal sources of electrostatic charging anddischarging. The first and most common source to date of electrostaticdischarge (ESD) is that due to human handling. The second source of ESDis that which takes place in automated test and handling systems. Theequipment can accumulate static charge due to improper grounding, whichis then transmitted through the IC when it is picked up for placement inthe test socket or carrier. The third possibility is that the IC itselfis charged during transport or because of contact with a highly chargedsurface or material. The IC remains charged until it comes into contactwith a grounded surface such as a large metal plate or a test socket. Itis then discharged through its pins and the large currents in theinternal interconnect can result in high voltages inside the device.These voltages can cause damage to the very thin dielectrics andinsulators present in the IC.

[0005] The three ESD mechanisms are known as (1) human body model (HBM),(2) machine model (MM), and (3) charged device model (CDM).

[0006] Failure modes for the HBM and the MM test methods are typicallyfound in the diffusion regions of the protection circuits. However, CDMfailures are usually gate oxide damage either at the pad, or in somecases internally in the circuit. The most common location for gate oxidedamage is in the pMOS transistor of the input buffer. The HBM testmethod is the most popular standard method, and an equivalent ESDcircuit for modeling HBM is shown in FIG. 1, and the discharge waveformof HBM is then shown in FIG. 2.

[0007] The high current, short duration pulse can be easily reproducedusing a charged transmission line. The advantage of the transmissionline pulse (TLP) tester is that a constant current pulse is generatedwhich enables the behavior of the protection structure under currentconditions to be studied. The double exponential and oscillating pulsesof standard ESD testers make it difficult to determine how theprotection structure operates. Hence, the TLP tester has become popularwith many ESD protection circuit designers and researchers intoprotection circuit operation and physics.

[0008] The TLP tester configuration has the advantage that it is easilydesigned to avoid sensitivity to internal parasitic elements. Hence, weobtain reproducible testing which can be correlated to HBM or MM typedischarges.

[0009] The relationship between TLP method and HBM can be described asthe following equation:

V _(ESD-HBM) =I _(t2) ×R _(HBM)  eq.(1)

[0010] wherein the V_(ESD-HBM) is the electrostatic discharge voltagemeasured by the human body method, I_(t2) is the second breakdowntrigger current, and the R_(HBM) is the resistance of a resistor used inHBM. R_(HBM) is set equal to 1500 ohm in the TLP method. After measuringthe second breakdown trigger current by TLP method, we will be able tocalculate an equivalent electrostatic discharge voltage which is assumedto be equal to that measured by HBM.

[0011] In the Eq. 1, the R_(HBM) is just a human body equivalentresistance, and the resistance of devices and leads is neglected.However, we found that the resistance of devices and leads issignificant; thus we introduce the parasitic series resistance, Rs, tomodify the Eq. 1.

SUMMARY OF THE INVENTION

[0012] It is an object of the invention to introduce a parasitic seriesresistance to modify the relationship between the transmission linepulse method and human body model.

[0013] It is another object of the invention to solve the parasiticseries resistance and electrostatic discharge voltages.

[0014] According to the foregoing objects, the present inventionintroduces a parasitic series resistance into the Eq. 1, and then theequation become

electrostatic discharge voltage=electrostatic discharge current×(thehuman body equivalent resistance+parasitic series resistance)

[0015] In present invention, electrostatic discharge voltages andparasitic series resistances can be obtained by using the least squareerror solution method.

[0016] Firstly, the second breakdown trigger current I_(t2) was measuredby transmission line pulse method, and the electrostatic dischargevoltage V_(ESD-HBM) was measured by human body model test method. Then,the sum of square S=Σ[the electrostatic discharge voltage measured byhuman body model test method V_(ESD-HBM)—the second breakdown triggercurrent measured by transmission line pulse method I_(t2)×(the humanbody equivalent resistance R_(HBM)+parasitic series resistance Rs)]²∘ Inorder to get a optimal value of Rs, the sum of square is minimized. Thenwe can apply the value of parasitic serial resistance into Eq.2 tocalculate the human body equivalent electrostatic discharge voltage ofTLP method (V_(ESD-TLP)).

[0017] Compare the human body equivalent electrostatic discharge voltageobtained from the TLP method with the electrostatic discharge voltagemeasured by HBM test method, we can find that the equation provided inthis invention is better than that provided in prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The foregoing aspects and many of the accompanying advantages ofthis invention will become more readily appreciated as the same becomesbetter understood by reference to the following detailed description,when taken in conjunction with the accompanying drawings, wherein:

[0019]FIG. 1 shows an equivalent ESD circuit for modeling HBM dischargewaveforms.

[0020]FIG. 2 shows the discharge waveform of HBM.

[0021]FIG. 3 shows an equivalent ESD circuit of TLP system.

[0022]FIG. 4 is a flowchart for explaining the method provided in thisinvention.

[0023]FIG. 5 show a Comparison between the electrostatic dischargevoltages obtained by conventional HBM and that obtained by TLP methodsaccording to Eq. (1).

[0024]FIG. 6 shows a Comparison between the electrostatic dischargevoltages obtained by conventional HBM and that obtained by TLP methodsaccording to Eq. (2).

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] In the prior art, the relationship between TLP method and HBM isaccording to Eq. 1, and the second breakdown trigger current can beobtained by TLP method. Then, the current is multiplied by the humanbody equivalent resistance (R_(HBM)) to get the human body equivalentelectrostatic discharge voltage (V_(ESD-TLP)) The result is shown in theFIG. 5. The R_(HBM) used in HBM is a constant which is set equal to 1500ohm. The FIG. 5 also shows the electrostatic discharge voltage(V_(ESD-HBM)) obtained by HBM test method and the relative differenceexpressed in percentage between the V_(ESD-TLP) and V_(ESD-HBM).

[0026] As shown in FIG. 5, we can find that the relative differencesexpressed in percentage between V_(ESD-TLP) and V_(ESD-HBM) areaveragely greater than 10%. Therefore, we think that the Eq. 1 can notdescribe the relationship between the TLP method and HBM. In addition,we note that the R_(HBM) in Eq. 1 is only a resistance of a resistor usein HBM, and the resistance of devices and lines are not considered. Sowe introduce a parasitic series resistance to modify the Eq.1. Thus, theEq.1 can be rewritten to Eq.2 as following

V _(ESD-HBM) =I _(t2)×(R _(HBM) +R _(S))  Eq. 2

[0027] The V_(t2) and Rs in Eq.2 are two independent variables, and theycan not be solved directly and exactly. But we can use the least squareerror solution method to find the optimal values. The first step of theleast square error solution method used to solve the Rs is to measurethe V_(ESD-HBM) by HBM test method and the I_(t2) by TLP method. Thenthe sum of square, S, is expressed as the Eq.3. When the sum of squareis minimized, the Rs can be expressed as Eq.4. Thus, after getting theV_(ESD-HBM) and I_(t2), we can calculate the Rs according to the Eq.4,and then calculate the the human body equivalent electrostatic dischargevoltage (V_(ESD-TLP)) derived form the TLP method according to Eq.2.$\begin{matrix}\begin{matrix}{S = \quad {{\sum\limits_{i = 1}^{m}\delta_{i}^{2}} = {{\sum\limits_{i = 1}^{m}\left\lbrack {V_{{ESD} - {HBM}_{i}} - V_{{ESD} - {TLP}_{i}}} \right\rbrack^{2}} =}}} \\{\quad {\sum\limits_{i = 1}^{m}\left\lbrack {V_{{ESD} - {HBM}_{i}} - {\left( {R_{HBM} + R_{S}} \right) \times I_{{t2}_{i}}}} \right\rbrack^{2}}}\end{matrix} & {{Eq}.\quad 3} \\{R_{S} = \frac{\sum\limits_{i = 1}^{m}{\left\lbrack {V_{{ESD} - {HBM}_{i}} - {R_{HBM} \times I_{{t2}_{i}}}} \right\rbrack \times I_{{t2}_{i}}}}{\sum\limits_{i = 1}^{m}\left( I_{{t2}_{i}} \right)^{2}}} & {{Eq}.\quad 4}\end{matrix}$

[0028] In accordance with the above method, we use gate-grounded NMOStransistor to be the protecting structure for ESD in this embodiment.The flowchart of this embodiment is shown as FIG. 4. Firstly, the secondbreakdown trigger currents, I_(t2), of samples to the number of m aremeasured by TLP method 10. And the electrostatic discharge voltage,V_(ESD-HBM), is measured by HBM test method 15. Each measured current orvoltage, as shown in FIG. 6, is an average of eight measurements.Secondly, the Rs can be calculated according to Eq.4 derived from theleast square error solution method 20, and the value of Rs is 182 ohm.Then, we can calculated the human body equivalent electrostaticdischarge voltage derived form TLP method V_(ESD-TLP) 30 by applying thevalue of Rs into the Eq.2. Finally, we calculate the relative differenceof the electrostatic discharge voltages measured from HBM test methodV_(ESD-HBM) and that derived from TLP method V_(ESD-TLP). All resultsare shown in FIG. 6, and we can find that the correlation of V_(ESD-TLP)and V_(ESD-HBM) in this invention is better than that in prior art.

[0029] In addition, the parasitic series resistance can also be obtainedby the simplified 4th order lumped element model. This model isdescribed as the Eq.5 and Eq.6, wherein the Ls is the parasitic seriesinductance and is set equal to 7.5 uH; the C_(HBM) is the capacitance ofthis model and is set equal to 100 pF. In theory, when the I(t) in theEq.5 equals to I_(t2), it will be the maximum as well as the f(t). Wecan obtain the parasitic series resistance from the Eq.4 and Eq.5, andit is 194 ohm in this embodiment. Then we can calculate the human bodyequivalent electrostatic discharge voltages derived from TLP method byEq.2, and the relative differences expressed in percentage between theelectrostatic discharge voltages measured from HBM test method and thatobtained from the simplified 4th order lumped element model. All theresults are also shown in FIG. 6. We can find that the results obtainedby least square error solution method are in accordance with thatobtained by the simplified 4th order lumped element model.

I(t)=V _(HBM)×ƒ(t)/(R _(HBM) +R _(S))  Eq. 5

[0030] $\begin{matrix}{{f(t)} = {\left( {1 - e^{\frac{{- {({R_{HBM} + R_{S}})}} \times t}{L_{S}}}} \right) \times \left( e^{\frac{- t}{{({R_{HBM} + R_{S}})} \times C_{HBM}}} \right)}} & {{Eq}.\quad 6}\end{matrix}$

[0031] Thus, we think that the TLP method is well correlated to HBM byintroducing a parasitic series resistance.

[0032] Although specific embodiments have been illustrated anddescribed, it will be obvious to those skilled in the art that variousmodifications may be made without departing from what is intended to belimited solely by the appended claims.

What is claimed is:
 1. A method for measuring a parasitic seriesresistance which is introduced to modify the relationship between thetransmission line pulse method and the human body model test method,comprising the steps of: determining a resistance of an intrinsicresistor which is used in human body model, wherein said resistance ofthe intrinsic resistor is set equal to a constant; measuring a firstelectrostatic discharge voltage by the human body model test method;measuring a breakdown trigger current by the transmission line pulsemethod; and determining said parasitic series resistance according tosaid resistance of the intrinsic resistor, said first electrostaticdischarge voltage, and said breakdown trigger current.
 2. The methodaccording to claim 1, further comprising a step of determining a secondelectrostatic discharge voltage which is derived from the transmissionline pulse method and equals said breakdown trigger current multipliedby a sum of said resistance of the intrinsic resistor and said parasiticseries resistance.
 3. The method according to claim 1, wherein saidbreakdown trigger current is a second breakdown trigger current.
 4. Themethod according to claim 1, wherein said step of determining saidparasitic series resistance is completed by using a least square errorsolution method.
 5. A method for measuring a parasitic series resistancewhich is introduced to modify the relationship between the transmissionline pulse method and the human body model test method, comprising thesteps of: determining a resistance of an intrinsic resistor which isused in human body model, wherein said resistance of the intrinsicresistor is set equal to a constant; measuring a first electrostaticdischarge voltage by the human body model test method; measuring asecond breakdown trigger current by the transmission line pulse method;and determining said parasitic series resistance by using a least squareerror solution method.
 6. The method according to claim 5, furthercomprising a step of determining a second electrostatic dischargevoltage which is derived from the transmission line pulse method andequals said second breakdown trigger current multiplied by a sum of saidresistance of the intrinsic resistor and said parasitic seriesresistance.
 7. A method for determining a electrostatic dischargevoltage derived from the transmission line pulse method, comprising thesteps of: determining a resistance of an intrinsic resistor which isused in human body model, wherein said resistance of the intrinsicresistor is usually set equal to a constant; measuring a firstelectrostatic discharge voltage by the human body model test method;measuring a second breakdown trigger current by the transmission linepulse method; determining a parasitic series resistance by using a leastsquare error solution method, wherein said parasitic series resistanceis introduced to modify the relationship between the transmission linepulse method and the human body model test method; and determining asecond electrostatic discharge voltage derived from the transmissionline pulse method by multiplying said second breakdown trigger currentby a sum of said resistance of the intrinsic resistor and said parasiticseries resistance.